The present invention relates to a gas sensor which is superior in response characteristic and is capable of being small-sized.
A sensor for various gases such as oxygen, hydrogen, hydrogen peroxide, ammonia, carbon dioxide and the like has been heretofore employed in a broad range of technical fields. An ordinary sensor separates a particular gas component from mixed gases to detect the concentration of the particular gas by means of a detecting apparatus. A gas permeable membrane which selectively lets the particular gas pass through is employed for the separation of the mixed gases, and the detection of the particular gas in the mixed gases is carried out by, for example, measuring electric energy after the concentration of the separated gas is converted into the electric energy. FIG. 1 is a vertical cross-sectional view showing an example of a conventional gas detecting sensor. The sensor consists of an upper curved permeable membrane A which lets a specific gas pass through; a platinum electrode E which consists of a curved portion B being substantially the same shape as the permeable membrane A, a horizontal portion C inwardly extending from the base portion of the curved portion B and a cylindrical portion D connected with the inner edge of the horizontal portion C; an insulated portion F disposed on the outer surface of the horizontal portion C and the cylindrical portion D of the platinum electrode E; and a counter electrode G surrounding the lower portion of the insulated portion F. An electrolyte H is filled in the space between the permeable electrode A and the curved portion B and in the space around the insulated portion F.
When the conventional sensor having the thus mentioned construction is employed, mixed gases are provided to the permeable membrane A of the sensor from the upper space as shown by the arrows. A specific component in the mixed gases, for example, hydrogen selectively permeates the permeable membrane A to be dissolved in the electrolyte H so that the dissolved hydrogen reaches the curved portion B of the platinum electrode E on which the hydrogen is ionized to a hydrogen ion. The ion reaches the counter electrode G by the movement of the electrolyte H, and the hydrogen concentration in the mixed gases is detected by measuring the voltage generated between the platinum electrode E and the counter electrode G. The sensor of this type is called a voltage type. On the other hand, a current type is also known in which oxidation or reduction of the detected gas is positively carried out on the platinum electrode and the detected gas concentration is determined by measuring the current generated during the oxidation or reduction.
Since, however, the moving speed of the specific gas in the electrolyte H between the permeable membrane A and the curved portion B is quite slow when either process is employed, a relatively long period of time is required before the gas is detected. When the membrane A and the curved portion B are brought closer in order to shorten the time required, the resistance against the movement of the electrolyte H from the space therebetween to the counter electrode G increases. In any case, the sensor has the disadvantage of being inferior in the response characteristic.
Further, conventional sensors including one shown in the attached drawing are generally large so that a smaller-sized sensor is demanded to meet the requirement of the decrease of the installation area.